1. Field of the Invention
This invention relates to a support of aluminum alloy used for a lithographic printing plate and more particularly, it is concerned with an aluminum alloy support for a lithographic printing plate, which is excellent in fatigue resistance, thermoplastic property and printability.
2. Description of the Prior Art
Lithographic printing plates which have generally been used are obtained by submitting the so-called presensitized printing plates (which will hereinafter be referred to as "PS plates") to plate making processings such as steps of imagewise exposing, developing, gum coating and so on. The PS plates are prepared by coating light-sensitive materials onto aluminum plates having the surfaces subjected to surface treatments such as surface roughening and anodic oxidation treatments and then drying them. It is well known that an area where the light-sensitive layer remains undissolved after the above described developing step forms an image part and the other area where the light-sensitive layer is removed to expose the underlying aluminum surface becomes water-acceptable and thus forms a non-image part because of being hydrophilic.
As the support for such a lithographic printing plate, there have generally been used aluminum plates of light weight, excellent in adaptability to surface treatments and machining as well as corrosion resistance. Conventional materials used for this purpose are aluminum alloy plates with a thickness of 0.1-0.8 mm according to JIS A 1050 (Al alloys of a purity of at least 99.5 wt %), JIS A 1100 (Al-0.05-0.20 wt % Cu alloys) and JIS A 3003 (Al-0.05-0.20 wt % Cu-1.5 wt % Mn alloys) which surfaces are roughened by either or at least two of mechanical, chemical and electrochemical treatments and then subjected to anodic oxidation.
More specifically, there have hitherto been proposed aluminum lithographic printing plates which are subjected in sequence to a mechanical surface roughening treatment, chemical etching treatment and anodically oxidized film forming processing, as described in U.S. Pat. No. 3,834,998; which are subjected in sequence to a chemical etching treatment and anodically oxidized film forming processing, as described in Japanese Patent Application OPI (Kokai) No. 61304/1976; which are subjected in sequence to an electrochemical treatment, aftertreatment and anodically oxidized film forming processing, as described in Japanese Patent Application OPI (Kokai) No. 146234/1979; which are subjected in sequence to an electrochemical treatment, chemical etching treatment and anodically oxidized film forming processing, as described in Japanese Patent Application OPI (Kokai) No. 28123/1973; and which are subjected to a mechanical surface roughening treatment and subsequently to the treatments described in Japanese Patent Application OPI (Kokai) No. 28123/1973.
Up to 100,000 sheets of clear prints can be obtained by providing a suitable light-sensitive layer on such a support, but it is still desired to obtain a further great number of prints from one printing plate (improvement of printing resistance). To this end, it is effective to subject a PS plate using an aluminum alloy plate as a support to exposure, development and heat treatment at a high temperature, i.e. so-called burning treatment, thereby strengthening an image part, as described in detail in Japanese Patent Publication Nos. 27243/1969 and 27244/1969. In this burning treatment, the heating temperature and time, depending upon the variety of the resin used for forming the image, are generally 200.degree. to 280.degree. C. and 3 to 7 minutes.
Lately, it has been required that this burning treatment is carried out at a high temperature in a short time so as to shorten the time for the burning temperature. When the commonly used aluminum alloy plates are heated at a high temperature, e.g. 280.degree. C. or higher, however, recrystallization of aluminum takes place, the strength thereof is extremely lowered and the printing plates are not firm, thus resulting in drawbacks that handling thereof is very difficult and it is impossible to set in printing machines or to subject to registering in multicolor printing. Therefore, the demand for a support consisting of an aluminum alloy plate which is stable and excellent in heat resistance is increasing.
Lately, printing speeds have been increased with the progress of the printing technique. Accordingly, this causes an increase in the stress applied to the printing plate which is fixed mechanically to both the ends of a rotating cylinder installed in a printing machine and when the strength of the aluminum printing plate is insufficient under the increased stress, there occur deformation or break of the printing plate at the fixed parts resulting in a shear in printing and cut of the printing plate due to repeated stress at the bent part thereof, whereby to make it impossible to continue the printing operation.
Aluminum alloy plates of the prior art according to JIS A 1050 can provide a uniform rough surface and suitable surface roughness in an electrochemical surface roughening treatment and can avoid strains on a non-image area during printing, but are inferior in fatigue resistance and heat softening resistance. On the other hand, aluminum alloy plates of the prior art according to JIS A 3003 have a sufficient fatigue resistance and heating softening resistance, but meet with the disadvantages that a uniform rough surface and suitable surface roughness are hardly obtained by an electrochemical surface roughening treatment and stains tend to occur on a non-image area during printing.